With a support document (via Dan Moren at Six Colors), Apple has detailed the Wi-Fi Assist feature of iOS 9, which automatically switches to cellular data if a Wi-Fi connection is performing poorly. Some of the interesting tidbits:

Wi-Fi Assist will not automatically switch to cellular if you’re data roaming.

Wi-Fi Assist only works when you have apps running in the foreground and doesn’t activate with background downloading of content.

Wi-Fi Assist doesn’t activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.

Wi-Fi Assist received a bit of criticism when iOS 9 launched as some users couldn’t figure out why their cellular data usage increased. I’m glad that Apple has shared more details on the subject, though I still think it should be off by default.